Friday, March 7, 2008

Dishwashing Detergent

Did you know that powder is more economical than liquid. Do your homework. It may take time and some careful math, but figuring out which products offer the best bargain for your specific need will help you shop more wisely.
For instance, we were curious about automatic dishwashing detergent. Like many other products, the quantity used on the package labeling indicates the net weight of the product inside. In most cases, you can divide the product's price by the weight in order to determine the "cost per unit" or "unit cost," making it easier to compare the price of one package with the price of another.
The shortcoming of this method, however, is that weight does not always translate into a meaningful measure of use. In the case of automatic dishwasher detergent, we wanted to know whether liquid or powder offered the better value; but dishwashing machines use a pre-determined volume of detergent. That is to say there is a little measuring cup-like cavity built into the door of the dishwasher. You fill that space with detergent and press start.
The trick is that liquid weighs more than powder for any given volume in most cases, and this is true with the detergent in question. Because of that difference, you can not assume that the detergent with the lower unit cost is the best value. Instead, you must figure out the "cost per use." Since 32 oz of powdered detergent provide more washes than 32 oz of liquid detergent, the powdered detergent has a built-in price advantage that must be overcome by a lower unit cost if the liquid detergent is to be cheaper.
For the liquid and powder we compared, the liquid would need to be priced at least 7% less than the powder before it became the better buy. Our math is provided below in case you want to do a similar test on your own.
Our washer is built to use about 2.5 fluid ounces (that's volume) of detergent per wash. That volume of the liquid detergent weighs 3.5 ounces (that's weight). Therefore a 65 oz bottle of liquid detergent yields about 18.5 washes (65 divided by 3.5 = 18.57).
The same volume of powdered detergent weighs only 3.25 ounces. So, a 65 oz box yields about 20 washes (65 divided by 3.25 = 20).
The powdered detergent provides 7% more washes per unit than does the liquid detergent (20 divided by 18.57 = 1.067). As such, the powdered detergent enjoys a 7% price advantage when compared to liquid detergent. In order for the liquid detergent to become the better value, its price per ounce must be at least 7% lower than the price per ounce of the powdered detergent.
Though we haven't tested it yet, we suspect the same holds true for laundry detergent.

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1 comment:

Anonymous said...

I LOVE the title of your blog!! Thanks for this cost analysis...very interesting!! We have used liquid dishwasher detergent since moving out to the country, because somewhere I heard that powder isn't good for a septic system. Have you heard that? I don't know, it may worth trying, since it will save us money!!!! :)
Have a blessed day!!